CD4+ T-cells play a key role in the induction and regulation of immune responds. These cells recognize peptides complexed with class II molecules on the surface of antigen-presenting cells (APC). The majority of MHC class II molecules are occupied by peptides derived from "self" proteins. T-cell development in the thymus involves interaction of Ag-specific T-cell receptors (TCRs) with MHC molecules expressed by thymic cortical epithelial cells and bone marrow derived APC. The engagement of TCR by self MHC in the thymus may lead to positive selection of self-MHC-restricted T-cells, or to negative selection, resulting in deletion of immature T-cells bearing TCRs with a high affinity for self peptide:MHC. The lack of appropriate MHC ligands for positive selection causes arrest of further differentiation of thymocytes followed by cell death. The overall goal of this project is to understand the role of "self" peptides in the development of CD4+ T-cells three parts. First, what are repertoires of self peptides bound to MHC class II molecules expressed in thymic cortical epithelial cells and in bone marrow derived APC? Second, what is the role of the class II:peptide repertoire in positive selection of T-cells in vivo? Third, what is the role of specific "self" peptides in the positive selection of T-cells? To carry out these studies they will build an extensive database of "self" peptides associated with a specific class II molecule and evaluate their contribution to the selection of CD4+ T-cells. These will be facilitated by the use of TCR transgenic mice and monoclonal antibodies specific for two define peptide:class II complexes in combination with mutant mice with severely altered repertoires of "self" class II binding peptides.